The invention relates to a hydrocarbon transfer system comprising a first structure having a length direction, a width direction and a deck level, a support structure extending upwardly from deck level of the first structure and supporting a track extending in the transverse direction, a movable frame part being connected to the track, a substantially transverse member and a vertical member being attached to the movable frame part, a vertical member extending downwardly from a first end of the transverse member from a movable joint such as to be pivotable around a first axis extending in the length direction and a second axis extending in the transverse direction
Such a hydrocarbon transfer system is known from EP-A-1 389 580. In the known system, a vertical tower is attached to a submerged production/storage vessel. A transverse manipulator arm is attached to the tower and has telescoping arm parts that can move transversely with respect to the vessel. A vertical suspension member extends downwardly from the free end of the manipulator arm and can rotate around an axis extending in the manipulator arm direction and around a second axis extending in a length direction of the vessel. The lower end of the suspension member carries a structural connector for attaching to receiving vessel moored alongside the storage vessel. A flexible hose extends from the tower to the lower end of the suspension member for attaching to fluid transfer ducts on the receiving vessel.
The known transfer system has as a disadvantage that the storage vessel is submerged below water level and that the deck is not accessible. Furthermore, the flexible fluid transfer duct extends across a relatively large distance and occupies the space between the far side of the storage vessel and the receiving vessel, hence hampering deck access even in case the deck level would be raised above water level. The curved flexible transfer duct is furthermore subject to uncontrolled swinging motions caused by wind and by wave movements which may result in undesired forces on the points where the flexible duct is connected to the tower and to the structural connector at the end of the vertical suspension member. The first structure may be a quay, tower, barge, vessel or the like.
Another hydrocarbon transfer system of the above-mentioned type is known from WO 2005/105565 A1 which shows a first vessel for containing hydrocarbons and hydrocarbon transfer means which are connected to a tank on the first vessel. The hydrocarbon transfer means comprise a connecting member for connecting to a second vessel which is moored at a relative large distance of for example 25 m or more alongside the first vessel. The hydrocarbon transfer means bridging the large gap between the two structures comprise a frame for carrying the fluid transfer duct with a connecting member at one of its ends. Such a large distance mooring arrangement between two structures is known from unpublished patent application EP051042182 “Soft quay mooring” in the name of applicant.
The known hydrocarbon transfer system has as a disadvantage that when the connecting member is connected to the second vessel, stress is created in the fluid transfer duct and/or the frame because of movement of the moored second vessel relative to the first vessel. As the transfer ducts need to bridge a large gap of more than 25 m between the two structures which are moving relative to each other, large forces and moments are introduced in the transfer system bridging the gap. The end of the transfer ducts will need to follow the movements of the second structure which creates a motion envelope for the connector in which the system must be able to function correctly and safely. The combination of large distance, large dimensions of the transfer system and motion envelope creates inertia related fatigue problems within the transfer system. On top of the motion envelope there will be a relative large draft variation (up to 5 m) between the two structures during the offloading off LNG from one structure into the other structure as in the case of two floating structures one will rise from a loaded draft level to an unloaded draft level while the draft level will increase. In addition to this, there are relative movements between the structures even when a vessel is moored alongside a static structure, like a quay. One of the movements of a moored second structure is a sway motion or roll motion in the direction from and towards the first structure alongside which the second structure is moored. The know transfer system compensates that movement by a vertical transfer duct part which is connected to the frame pivotable around an axis extending in the length direction. Because of the pivoting displacements of the vertical transfer duct part, also an additional up and down displacement of the connecting member relative to the first structure is created. This up and down movement of the connection member is in the height direction and creates stress in the fluid transfer duct and/or the frame. Stress in the fluid transfer duct and/or the frame can create leakage of the transferred materials. Because the hydrocarbon transfer system is used for transferring highly inflammable hydrocarbons, such as LNG, leakage must at all times be avoided. Therefore the stress in the fluid transfer duct and/or the frame of the hydrocarbon system must be brought to a minimum.
A further disadvantage of the known hydrocarbon transfer system is that because of the pivoting movement of the vertical transfer duct around the axis extending in the length direction, large displacements of the moored second structure from and towards the first structure can not be compensated.
Another disadvantage of the known hydrocarbon transfer system is that it cannot function correctly over such a large distance if there is a variation in the position of the connection points or flanges on the second structure, as the motion envelope of the end of the transfer duct will be completely different. Another disadvantage related to the large distance between the two structures and the large variations in draft (up to 5 m) between the two structures during offloading of LNG is that the known loading arms can not provide the same motion envelope of the connector at the end of the fluid duct which is needed in all circumstances.